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In order to explore the law of proppant migration and placement under clean fracturing fluid, an Euler multiphase flow model was established through FLUENT numerical simulation software based on computational fluid dynamics(CFD). The model takes into account the rheological properties of fracturing fluid and the friction and collision of proppant particles. The influence of proppant size, density and sand ratio on the shape of the sand dike is studied by using the equilibrium height and equilibrium time as the characterization parameters. The results show that the clean fracturing fluid is easier to carry the proppant to the depth of the fracture than the conventional fracturing fluid, so that the proppant is evenly distributed in the fracture; proppant with large particle size and high density can easily settle in the fracture opening under the carrying of the clean fracturing fluid, while proppant with small particle size and low density can improve the sand carrying capacity of fracturing fluid and transport proppant into deep fracture. Increasing sand ratio in a certain range can improve the height and laying area of the sand dike, but excessive sand ratio has little effect on the shape of the proppant sand dike and is easy to cause sand blockage.

Nano-clean fracturing fluids have broad application potential in coalbed methane reservoir fracturing owing to their high stability, good temperature resistance, low filtration loss, and strong frictional resistance reduction. However, the sand-carrying regularity of nano-clean fracturing fluids in coalbed methane reservoirs is unclear, especially for complex fractures with variable directions. This study established a sand transport model that considers proppant collision, wall friction blocking, fracture fluid filtration loss, and the fracture branching angle to study the sand-carrying law of nano-clean fracturing fluids and its influencing factors in complex fractures. The degrees of influence on equilibrium height and placement rate from high to low were the proppant particle size, proppant density, fracturing fluid properties, sand ratio, and pumping discharge volume, and the correlation degrees obtained by grey correlation analysis are 0.862, 0.861, 0.855, 0.854, and 0.832, respectively. As the complexity of the fractures deepens and the resistance increases, the flow rate of the fracturing fluid is reduced, making it difficult for the proppant to enter the branching joints. The sand-carrying performance of a nano-clean fracturing fluid is better than that of a common clear-water fracturing fluid. The fluid-structure coupling model of a nano-clean fracturing fluid can accurately characterize the sand-carrying law of nano-clean fracturing fluids, providing a research basis for optimizing high efficiency sand-carrying fracturing fluid parameters in coalbed methane reservoir fracturing construction. [ABSTRACT FROM AUTHOR]

Air leakage in borehole not only seriously restricts the development and utilization of coal bed methane (CBM) but also affects gas disaster prevention and underground safety mining. Several triaxial-seepage experiments were conducted to analyze the seepage characteristic change of raw coal under different loads in consideration of the above-mentioned problems. On this basis, a dynamic multifield coupling model, including coal permeability evolution, creep mechanical, and binary gas flow system, was proposed to describe the CBM drainage process. Based on this fully coupling model, factors influencing air leakage flux were investigated by numerical method. Findings show that the dynamic change of air leakage flux is determined by stress and permeability variation. Therefore, in practical work, grouting method can be adopted to decrease coal permeability and softening modulus as well as increase coal residual strength and support stress, so as to reduce air leaking in borehole. Furthermore, we also proposed a new remedy method, including self-developed GP material and corresponding sealing–plugging integrative device, for preventing borehole leakage. The field test results indicate that the gas concentration of the boreholes was effectively enhanced after the leakage treatment by this method, and a great drainage level was still maintained at the late period. The results in this work can provide a remedy method for improving borehole sealing and drained gas quality. Such method is important for enhancing the CBM utilization, reducing greenhouse gas emission, and ensuring mining safety. [ABSTRACT FROM AUTHOR]

To solve the serious filtering problems of fracturing fluid during active hydraulic fracturing of coalbed methane, which blocks gas migration channels with residual fracturing fluid and results in a rapid decline in coalbed methane production, several preliminary experiment of thermal nitrogen injection was carried out. To explain the process and mechanism of thermal nitrogen injection at the nanoscale, an adsorption and diffusion model was established to study the adsorption and diffusion of methane, water and fracturing fluid during thermal nitrogen injection. The results showed that the self-diffusion coefficient of methane was increased by 23%, 45% and 32%, the self-diffusion coefficient of water molecule was increased by 108%, 185% and 138%, the self-diffusion coefficient of fracturing fluid molecule was increased by 169%, 275% and 181%, and the total number of atoms in the slit hole was decreased by 15%, 25% and 37%, respectively, when hot nitrogen was injected at 2, 4 and 6 Mpa, respectively. This shows that the numbers of methane, water and fracturing fluid molecules in the slit hole were significantly decreased by the thermal and displacement effects of hot nitrogen injection. The self-diffusion coefficient was significantly improved, and the effects of plug removal was remarkable. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is one of the most harmful natural disaster in coal mine production. The frequency and scope of outburst are closely related to the metamorphic degree of coal seam, the depth of coal seam, and the complexity of geological structure. In general, the use of traditional along‐seam borehole or cross‐seam borehole can meet the needs of gas control. However, for the coal seams with high outburst risk, these methods fail to meet the demands of high‐yield and high‐efficiency coal recovery as well as safe, green, and economical coal mining. Although pressure‐relief gas extraction in the protective seam and coalbed methane (CBM) pre‐extraction can eliminate outburst risk in coal seams, these methods can hardly help realize the green development trend of coal and CBM co‐extraction because they cannot guarantee the current situation of highly mechanized mining. In this study, a three‐dimensional coal and CBM co‐extraction strategy was proposed. The strategy includes three levels of three‐dimensional gas prevention and control methods. The first level is the combination of ground and underground gas extraction methods, to achieve joint extraction of ground and underground gas. The second level is the full‐coverage extraction of the coal resources to be mined, to achieve joint extraction through upper and lower cross‐seam borehole and along‐seam borehole. The third level is the optimization of ground and underground extraction and utilization systems, to realize the full utilization of CBM and to achieve the goal of coal and CBM co‐extraction. The three‐dimensional gas prevention and control method constructs a new gas control model for single thick coal seams with high gas outburst risk and realizes the all‐round coordination and unity of different gas control methods spatially and temporally. The gas control strategy completely eliminates the high gas outburst risk of a single thick coal seam and has been successfully applied in Hudi Coal Mine. The application results verify that the method can effectively guarantee the operation safety during highly mechanized mining, realize extraction and full utilization of separate‐source gas, and provide guidance and direction for coal and CBM co‐extraction with similar extraction conditions. [ABSTRACT FROM AUTHOR]

This paper aims to study the influence of molecular structural difference between deformed soft and hard coal on methane adsorption and obtains the quantitative indexes that reflect the above influence degree. In this paper, XRD and FTIR experiments were adopted to determine the molecular structure of typical deformed soft and hard coals with different ranks. The molecules structural parameters of deformed soft and hard coal were compared and analyzed. Based on the above structural difference characteristic, four corresponding physical models of macromolecular structure are established. A simulation for deformed soft and hard coals was established to test out the behavior difference of adsorption and its mechanism; by using Materials Studio (MS) software, same scenario is also applied on the other four physical models listed above. Then, the isothermal adsorption methane experiments of four kinds coal samples were carried out to verify the above simulation results. The results show that the molecular structure of deformed soft and hard coals has significant differences, i.e. comparing with hard coal, deformed soft coal has smaller interlayer spacing d 0 0 2 ; greater lateral sizes L a, stacking heights L c and the crystal nuclei sizes L a/ L c. Deformed soft coal had a more orderly structure crystal nuclei, bigger crystal volume, the larger basic structure unit (BSU) and more flattened. Deformed soft coal has less oxygen-containing functional groups, lower fatty hydrocarbon content and more C = C. These findings verified that dynamic metamorphism promotes aromatization and large molecular polycondensation of the coal, thus improving the aromatic degrees of the crystal nucleus. Due to the increasing of the lateral sizes being beneficial and the stacking heights being not conducive to the adsorption of the coal, the L a/ L c was proposed as an indicator of the coal adsorption methane ability. The isothermal adsorption experiments show that the adsorption capacity of deformed soft coal with the same rank is greater than that of hard coal, which is in good consistency with the above research results. Also, the above experimental results provide a theoretical basis for the determination of coalbed methane content and the estimation of CBM. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, coal mines in China have seen accidents caused by sudden release of hydrogen sulfide (H2S), with higher severity. Based on recent local and overseas research on the origin of hydrogen sulfide and the comparative analysis of isotopes, the paper presents three formation models of hydrogen sulfide: bacterial sulfate reduction (BSR), thermo-chemical sulfate reduction (TSR) and igneous activity, as well as the characteristics for identification. This paper analyzes and categorizes the formation models of some hydrogen sulfide in coal seams in China. Some suggestions were put forward for future research of H2S in coal mines in China. [Copyright &y& Elsevier]